I know it shows the item you have selected and almost acts
like a grouped item and the hierarchy of that item. But sometimes,
and I don't know how this happens, I can keep clicking and clicking
that item and more things appear in this space and I don't know how
that happens (for example I just saw "tween 1" appear there).
Undoing never gets rid of those, I have to revert to save.
I'm probably not explaining this good but I guess if I knew
what it was called I can do more research on it.
It displays a breadcrumb path from your root Stage to
wherever you are inside of a symbol. For instance, the image you
linked tells me that you are on a scene called "hvr950" and are
editing the graphic symbol "hvr950 title" If you click on "hvr950"
you will end up back at the root, or if you go into the edit mode
of a symbol inside of "hvr950 title" then that symbol will appear
in the list. (IE "Tween1" is probably a graphic that is inside of
"hvr950 title" and is titled in the manner of a graphic symbol
automatically created for you by Flash to perform a Motion Tween on
a shape. Double clicking on that graphic symbol will add "Tween 1"
to the list -- "hvr950" | "hvr950 title" | "Tween 1" -- which
displays the list of symbols back to your Scene's stage)